Engine systems may be configured with exhaust gas recirculation (EGR) systems via which at least a portion of the exhaust gas is recirculated to the engine intake. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, flow rate, various temperature, pressure, oxygen, and humidity sensors. Since the accuracy of the EGR estimation relies on the correct functioning of the various sensors, periodic sensor diagnostics are used.
However, humidity frequently remains constant over the course of a drive cycle. If humidity does change, it can be hard to determine if the humidity changed due to a change in temperature, the presence of rain, a car wash, or if the change is due to sensor degradation. Thus, performing a rationality check to test humidity sensor function may be difficult.
One example approach for diagnosing a humidity sensor is illustrated by Xiao et al. in U.S. Pat. No. 7,715,976. Therein, humidity sensor degradation is determined based on a comparison of an intake humidity estimated by a first humidity sensor in the intake manifold with an exhaust humidity estimated by a second humidity sensor in the exhaust manifold and an ambient humidity estimated by a third humidity sensor located outside of the engine. The sensor readings are compared during conditions when all the sensor readings are expected to be substantially equal, such as during engine non-fueling conditions in which the EGR valve is closed. If the readings of the three humidity sensors differ by more than a threshold, humidity sensor degradation may be determined.
The inventors herein have identified a potential issue with such an approach. The accuracy of determining degradation of any one humidity sensor may depend on the proper functioning of the other humidity sensors. Further, multiple humidity sensors may not be needed for engine control, and thus additional humidity sensors may not be available for comparison.
Accordingly, embodiments for diagnosing a humidity sensor based on correlation to output from an intake gas composition sensor are provided. One example method comprises indicating degradation of a humidity sensor based on a humidity sensor output and output from an intake gas sensor.
In this way, output from the intake gas composition sensor may be used to determine if the humidity sensor is degraded. For example, during selected conditions, when humidity of the intake air entering an engine increases, the oxygen concentration of the air decreases. This is because the water vapor displaces air in the volume of intake air entering the engine, thus lowering the oxygen concentration of the intake air. If the humidity sensor detects an increase in humidity and the intake gas composition sensor does not detect a corresponding decrease in intake air oxygen concentration, the humidity sensor may be erroneously determining that the humidity has changed, and thus degradation of the humidity sensor may be indicated.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.